Electrophotographic printers produce images by depositing toner on receivers (or “imaging substrates”), such as pieces or sheets of paper or other planar media, glass, fabric, metal, or other objects. Printers typically operate using subtractive color: a substantially reflective receiver is over-coated image-wise with cyan (C), magenta (M), yellow (Y), black (K), and other colorants. Other toner compositions can also be used to produce effects beyond simple image appearance.
In electrophotography, there is a need to deposit white toner in combination with colored toner for various purposes such as image quality and the like. The prior art discussed below deposits white ink and other color toner on the receiver.
For example, U.S. Patent Publication 2009/0220695 A1 discloses a method of creating a record medium using an ink jet process by which a non-white background can be completely hidden. This is achieved by printing a metallic ink first and then a white pink. Wherever there is an overlap between the two layers, an opaque layer is formed which completely hides the background color or transparency of the medium. A combination of metallic and white layers creates the opaque layer which is extremely white because of the scattering by the white layer and reflecting properties of the metallic layer.
U.S. Patent Publication 2011/0234660 A1 discloses a method of printing on a transparent medium by IJ process using color inks, metallic ink and white ink. The opaque areas are created by the process described in the '695 disclosure above. Use of white and metallic provides the cost advantage as well as be able to provide the desired luster effects. The image is viewed from the non-printed side for transparent substrate where the white layer is uniformly applied farthest from the medium. From opaque medium, white is applied first and then metallic and finally the color inks are jetted. The metallic layer serves as a specialty gloss layer to provide different effects and opacity.
U.S. Patent Publication 2013/0145383 A1 discloses an inkjet recording method which uses a white overlaying layer. The process is designed for remote proofing in which a longitudinal film is passed through two separate IJ stations. The substrate may contain an ink reception layer.
If the substrate is opaque, white is first laid down uniformly and after white layer is dried, color image is applied above it and dried again. On the other hand, when the substrate is transparent, color image is applied first and then dried. This is followed by the uniform application of white inkjet drops over the entire color image areas which are then dried again. In another variation, the white can be applied on the opposite surface in the case of a transparent substrate. Because the white is inkjet based, the preferred pigments are hollow or porous to avoid settling of heavy titania based white pigment. It further discloses an “inverse” type white ink application [0054 and 0057]; however, the white usage is based on total amount allowed by the substrate.
Although satisfactory, in U.S. Patent Publication 2009/0220695 A1, there is no adjustment of the white laydown with respect to the subsequent color inks, and two layers or more layers are required to create this opaque image. In U.S. Patent Publication 2011/0234660 A1, which is an inkjet process, there is no control of white ink based on color ink density; white is printed farthest from the viewing side, behind colors, not alongside. In U.S. Patent Publication 2013/0145383 A1, two printing stations are used, not one printing station, and the total white amount can exceed the total non-white amount ink. The present invention includes the advantages of adjusting the white laydown relative to color toner layers which reduces total toner cost, preserves the possible special visual effect provided by specialized substrates such as metallic/pearlescent substrate, and optimizes printable color gamut.